1. Field of the Invention
The invention relates to a condenser micro-phone apparatus for converting an acoustic vibration into an electric signal and, more particularly, to such an apparatus having an impedance converting device therein.
2. Description of the Related Art
Generally, a condenser microphone apparatus comprises a condenser microphone unit, a microphone signal output transmission line, a load resistor, a power source, and the like.
Hitherto, the condenser microphone apparatus has a problem such that when it is used in a cellular phone or the like, noises are generated due to radiation of a high frequency signal from a transmitting unit. For a countermeasure against such a problem, as a condenser microphone apparatus, a method whereby a bypass capacitor is provided between a source and a drain of an internal FET (field effect transistor) has been known.
First, a construction of a condenser microphone unit in the conventional condenser microphone apparatus will be described with reference to the drawings.
FIG. 1A is a cross sectional structural view of a conventional condenser microphone unit in which a bypass capacitor is provided between a source and a drain of an FET. In FIG. 1A, the condenser microphone unit comprises: a cotton 11 for preventing the dust from entering the microphone unit; a sound input hole 12; a metal casing 13 also serving as a shield; a movable electrode 14 which vibrated in response to an acoustic vibration; a movable electrode ring 15; a spacer 16; a fixed electrode 17; an insulator 18; an FET 19; a wiring circuit board 20; a bypass capacitor 21; a microphone signal output terminal 22; and a microphone common output terminal (ground terminal) 23.
The movable electrode 14, fixed electrode 17, and spacer 16 form a capacitor. Each of the movable electrode 14 and fixed electrode 17 is made of an electret material itself or an electret material itself is adhered to such an electrode and charges are accumulated on the surface of the electret material. The spacer 16 insulates the movable electrode 14 and fixed electrode 17. The movable electrode ring 15 supports the movable electrode 14. The insulator 18 supports the fixed electrode 17 while insulating it. The FET 19 buffer-amplifies a voltage that is generated in the capacitor comprising the movable electrode 14 and fixed electrode 17 and has a device (diode) for bias setting therein. The wiring circuit board 20 also seals a rear surface while wiring a circuit. The bypass capacitor 21 is a capacitor for bypassing a high frequency signal entering from the outside to a common output terminal.
FIG. 1B is a bottom view of the condenser microphone unit. Since it has a co-axial, the microphone signal output terminal 22 and microphone common output terminal 23 are concentrically arranged so as to be come into contact with each other even if the direction is not determined. There is another condenser microphone unit having pin terminals.
The operation of the conventional condenser microphone apparatus will now be described with reference to FIG. 2. FIG. 2 is a circuit diagram of the conventional condenser microphone apparatus.
A microphone signal output transmission line 31 is used for wiring on a mother board of an apparatus such as a cellular phone or the like. A decoupling capacitor 35 (a parasitic capacitor between layers of the microphone signal output transmission line 31 and a ground pattern is also included) is used to decrease the high frequency signal that is superimposed onto the microphone signal output transmission line 31. Similarly, a load resistor 32 and a power source 33 are arranged on the mother board of the apparatus such as a cellular phone or the like. The microphone signal output transmission line 31 is regarded to be connected to the ground in a high frequency manner at the decoupling capacitor 35 in terms of a circuit construction.
The high frequency signal is supplied to the microphone signal output terminal 22 through the microphone signal output transmission line 31 and applied to a drain of the FET 19. The high frequency signal is supplied to a gate of the FET through an electrostatic capacitance between the drain and gate of the FET 19, is AM-detected by a diode for biasing of the FET 19 or by a pn junction of a channel and the gate of the FET 19, generates a DC component, and is converted into noises in an audible band. In a high carrier frequency band of a radio apparatus, the microphone signal output transmission line 31 operates as an inductor and the bypass capacitor 21 operates as a serial circuit of an electrostatic capacitance and a parasitic inductance, so that a parallel resonance or a series resonance is caused at a specific frequency. For example, at a frequency of a series resonance due to the electrostatic capacitance and parasitic inductance of the bypass capacitor 21, since a voltage across the bypass capacitor 21 is small, a high frequency voltage that is applied to the drain of the FET 19 is small and no noise is generated at this frequency. On the other hand, a large resonance current flows at a series resonance frequency of the bypass capacitor 21 and microphone signal output transmission line 31. This is because an effective series resistance of the bypass capacitor 21 and microphone signal output transmission line 31 is very small. Therefore, the voltage across the bypass capacitor 21 increases, a high frequency voltage that is applied to the drain of the FET 19 is large, and large noises are generated from the condenser microphone apparatus even by a small amount of high frequency signal. As mentioned above, since a magnitude of the high frequency voltage that is applied to the drain of the FET 19 is largely changed depending on the frequency of the high frequency signal, it is difficult to keep such a frequency low in a wide band.
In the case where the condenser microphone apparatus with such a construction is used in a cellular phone or the like, an antenna and the condenser microphone apparatus are likely to be arranged at remote positions in terms of an apparatus construction. This is because it is necessary to arrange a handset at a position near the ear and arrange the condenser microphone apparatus to a position near the mouth and the antenna is arranged near the handset because a radiation efficiency is higher as the antenna is arranged at a position as high as possible. Further, a length of antenna is shortened due to a miniaturization of the apparatus and the realization of a high carrier frequency. In terms of the radiating characteristics of the antenna, the high frequency voltage is induced on the opposite side of the antenna and the high frequency voltage which is applied to the condenser microphone apparatus locating there is high. Also, a length of wiring is long and they are arranged so that the high frequency voltage is easily superimposed. Therefore, it is impossible to cope with such a situation merely by the bypass capacitor as a conventional countermeasure. Further, there is a cellular phone which is used by two frequency bands and it is necessary to take a counter-measure against the noises at a different frequency.